(CNN) -- The number of cases of Marburg hemorrhagic fever has continued to rise in northwestern Angola, the World Health Organization said Wednesday.

So far, 231 cases of the fever -- which is closely related to the deadly Ebola virus -- have been reported, the WHO said. Of those, 210 have died, a mortality rate of 91 percent.

Uige Province, where 190 of the 202 cases (94 percent) have proven fatal, has been the most severely affected area.

"The present outbreak of Marburg hemorrhagic fever is unprecedented in its size and urban nature, and its dimensions are still unfolding," the WHO said in a written statement Wednesday.

The security of health teams "remains a concern," and more vehicles are needed, the agency said.

"To bring the outbreak under control, the detection and isolation of patients needs to be much earlier, but this will not happen until the public understands the disease and the high risks associated with treating patients in homes," it added.

The agency applauded the decision by the International Federation of Red Cross and Red Crescent Societies to boost its presence in Uige. In collaboration with church and community leaders and traditional healers, the organizations' workers are going door to door to educate residents about the disease.

All evidence collected to date shows that casual contact plays no role in the spread of the virus, WHO said.

"Transmission requires extremely close contact involving exposure to blood or other bodily fluids from a patient who will most likely be showing visible signs of illness."

The virus can also be spread through contaminated bedding and clothing, it added.

Several cases have occurred among health-care staff members in Uige, though no foreign nationals other than those involved in care giving appear to have been stricken, it said.

The agency urged anyone considering travel to Angola to defer it, if possible.

Two cases have occurred in the capital city of Luanda, which has an international airport, raising the specter that the disease could spread.

Marburg has no vaccine or specific treatment.

In addition to Angola, outbreaks and sporadic cases have been reported in the Democratic Republic of Congo, Kenya and South Africa.

Marburg virus is named after the town in Germany where one of the first outbreaks was identified, in 1967.